CARBONATE MINERAL SATURATION STATE AND CRATONIC LIMESTONE ACCUMULATION

Sea-surface temperature/saturation data, sediment accumulation rates from tropical and temperatue shallow marine platforms, and paleolatitudes of ancient cratonic carbonate sequences have been complied to determine the influence of seawater chemistry on the distribution and rate of carbonate deposition. Tropical to subtropical accumulation rates are strongly dependent on the duration of time over which they are measured but, when considered at a time interval of 5 ky, range to a maximum mean rate of about 2 m/ky at a latitude of about 20-degrees and drop off precipitously between 25-degrees and 3-degrees-N. Accumulation rates exhibit a functional relation with respect to latitude and sea-surface saturation state and can be described as: R = k (OMEGA - 1)n, where R = accumulation rate, n = order of the ''process'', k = a rate constant, and OMEGA = calcite or aragonite saturation. Agreement between calcium carbonate accumulation rate data and rates calculated from heterogeneous-growth laboratory experiments of carbonate minerals suggests that spatial variation in physicochemical parameters of seawater composition is indistinguishable from biologic/ecologic factors in controlling areas and rates of shallow water carbonate accumulation in global oceans. Rate of shallow water carbonate deposition is strongly dependent on the equator to pole temperature/saturation gradient. Comparison of the latitudinal extent of Cretaceous shallow water carbonates with latitudinal limits of carbonate accumulation in modern settings indicates that Cretaceous limestone deposition occurred to latitudes some 10-degrees more poleward than at present. This difference suggests that mean annual Cretaceous seawater surface temperature gradient between the equator and 38-degrees north latitude was about 5-degrees-C lower (warmer) than that observed in modern oceans.